Hi Samuel, no adapter is needed with the 590S, it is made to fire mini shells without the adapter.
Stay safe & have a great day!

This newer model 590S was made with mini shells in mind my friend. No Opsol mini shell adapter needed at all to run the mini's.

no adapter needed "590S models cycle 1.75"; 2.75" or 3" shells, interchangeably, without the need for an adaptor"

Hi Samuel and welcome. This model will handle mini-shells without the adapter. For other 500/590 Maverick 88 models you would need this adapter :budsgunshop.com/product_info.php/products_id/36595/opsol+texas+miniclip+mini-clip+for+mossberg+500+590+12+gauge+1+round+polymer+black+finish -- Cheers and stay safe!

According to Mossberg, "590S models cycle 1.75"; 2.75" or 3" shells, interchangeably, without the need for an adaptor. Pick one shotshell length, or simultaneously load any combination of the three. "